Nonsmoking employees in trade positions, such as maintenance and repair workers, are significantly less likely than nonsmokers in professional fields to be protected against secondhand smoke, a new study finds.

Researchers from the Massachusetts Department of Public Health found 3 percent of nonsmokers in professional fields, such as doctors and nurses, software engineers, architects or teachers, said they were exposed to a co-worker’s tobacco smoke. Among trade workers who didn’t smoke, such as auto body workers, heating and air conditioning mechanics, cable TV installers and locksmiths, the rate of secondhand smoke exposure was 37 percent.

Among nonsmoking construction workers, 23 percent said they were exposed to secondhand smoke, while 20 percent of nonsmoking transportation workers, including taxi drivers, parking lot attendants and bus drivers, said they were exposed.

The study included data from surveys of more than 6,000 workers in Massachusetts between 2003 and 2010. Massachusetts passed a law in 2004 that bans smoking in all enclosed workplaces, the article notes. The year before the law went into effect, 8 percent of nonsmokers said they were exposed to secondhand smoke. By 2010, the rate of secondhand smoke exposure decreased to 5.4 percent.

The rate of smoking among workers in the state decreased from 18.5 percent in 2004, to 12.7 percent in 2010.

“Workers in these occupational groups may be in environments not covered by the workplace law, or they work in places where the law does apply but it’s difficult to enforce, like in vehicles,” lead author Kathleen Fitzsimmons told The Boston Globe. “Our findings suggest that comprehensive smoking bans work. But our results reveal there is more work to be done.”

The researchers presented their findings this week at the annual meeting of the American Public Health Association.